Transcript Work and Energy Problems

5-1 Energy and Work
• Everything you see and do involves energy
• Often you can’t see the energy, but can see the
effects…examples?
• If an object is able to change its environment, it has
energy.
• A baseball breaking a window has energy and does
work on the window causing it to move
• To do work we use energy to cause change
Defining energy
• Many different forms:
• radiant, electrical, chemical, thermal, nuclear
• Energy = ability to do work; to cause something to
move
• there is always a change when work is done so…
• Energy is the ability to cause change
On the move
• Kinetic energy = energy of motion; how much
energy depends on the mass and velocity of the
moving object. See fig 5-3 p 125
• Energy without motion is stored energy. This means
it has POTENTIAL to cause change.
• This is called potential energy(stored).
• Amount of potential energy depends on the objects
position or condition. Gravity gives objects their
“potential” to do work. Higher = more potential
Work ?
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Earning money? Yes, but not for science.
Transfer of energy through motion.
A force must be exerted through a distance.
And the force must be in the direction of the
movement.
• W=Fxd
• units are N x m = Joules or J
• James Joule; problems p 127
Decisions; decisions
• Two factors in determining whether work is done:
• something has to move; motion has to be in the
direction of the applied force!
• Carrying an object is NOT doing work!
• Swinging: involves changes in energy. See fig 5-7
p 128
Conservation
• Mechanical energy def p 129
• energy may change from one form to
another, but the total amount of energy
available remains constant = LoCoE
• energy is neither created or destroyed
• Look at the swing fig again…if energy isn’t
lost why does the swing stop? Where does
the energy go?
Where oh where has my energy gone?
• Friction/air resistance are acting on the
swing; changes the mechanical energy to
thermal energy(heat).
Human body
• We have several types of energy in our
bodies
• kinetic, potential, chemical, thermal
• we must take in energy(food,chemical) to
do work
• Calories: kilocalorie= 4180J
• see table 5-1 p 131
• section wrap up questions? Do Skillbuilder
Work Problems
• 1. A boy lifts his backpack 3 meters with a
force of 112 Newtons. How much work did
the boy do?
• 2. A girl lifts her dog 5 meters with a force
of 23 N. How much work did the girl do?
• 3. A suitcase weighs 55N. This kid picked it
up from the ground 3.5m high.How much
work is done???
more
• 4. A weightlifter lifts a 6,000N hand weight over
his head(2m) and throws it a distance of 5 meters.
Work-how much?
• 5. A girl threw a softball 30meters. The amount of
work done was 1620J. How much force did she
exert on the ball?
• 6. A person lifts a box weighing 25N from the
ground to the trunk of a car 2.4m. high. Work?
More
• 7. A girl lifts a box of books that weighs
24N and is lifted 3.5 meters to the top of a
shelf. How much work is done?
• 8. A man lifts a bucket 1.4 meters high with
a force of 60 N. What is the work being
done?
• 9. I lift ten 22kg bags of feed 1 meter and
carry them 9 m to the shed and drop them.
How much work?
more
• 10. A boy lifted a stack of boards 2m high with a
force of 14N. How much work is done?
• 11. A person lifts a crate with a force of 451N.
He lifts it 2.3 meters. How much work?
• 12. A girl lifts her books up .94 meters with a
force of 350N. How much work did she do?
• 13. A man’s trash can weighs 12N. He lifts it
into his truck which is 2m off the ground. How
much work is done on the trash can?
more
• 14. A girl lifts a full box that weighs 32 N
from the ground to a table that is 1 m high.
How much work is done?
• 15.A boy lifts his 25N dog off the floor
1.5m high to his bed. How much work is
done on the dog?
• 16. My backpack weighs 50N. I lift it from
my locker to my back at 2 m high. How
much work was done?
more
• 17. A grocery store worker lifts a box 3.2 meters
with a force of 115N. Work?
• 18. A bowling ball weighs 15N. A girl lifts it and
puts it on the shelf 3 meters high. How much work
is done?
• 19. A person lifts an object weighing 50N to a shelf
that is 2 m high. How much work was done.
• 20. A girl lifts 1600N; 2.7 meters. How much work
is done? How many kg is this? Pounds?